300 



'Popular Science Monthly 



of a single turn. Reference to Fig. 4 

 shows that each step to the left of switch 

 6^ adds in circuit ten turns of the coil, 

 and that these large jumps of inductance 

 may be filled in by the smaller steps se- 

 cured by moving switch i to the right. 



Fig. 5. Arco and Rendahl high power variometer 



This switching system has come into 

 wide use within the past few years. 



One of the difficult problems of radio 

 engineering is the construction of an 

 easily varied inductance capable of car- 

 rying such high currents as are encoun- 

 tered in the oscillation circuits of power- 

 ful transmitting stations. Flat spiral 

 coils have proved useful, but if they are 

 to be used for final adjustment some 

 way is needed to change their induct- 

 ance gradually without interfering with 

 the current through them. 1915 patent 

 No. 1131187, issued to G. von Arco and 

 R. H. Rendahl, shows an interesting way 

 of doing this. Referring to Fig. 5, two 

 sets of flat spiral coils a and h are seen 

 to be mounted in a framework which 

 permits the gr'oup a to be moved away 

 from the others by swinging them about 

 the pivots / as an axis. The whole set 

 of coils is connected in series, with taps 

 taken off at terminals k, r.nd the units 

 are carefully insulated from each other. 

 When the moving group is in the posi- 

 tion of closest coupling to the fixed coils 

 the system has its maximum inductance, 

 since the magnetic fields of all the coils 



are co-operating; when, however, the h 

 coils are swung out into the position in- 

 dicated by the dotted lines at the bottom 

 of Fig. 5, the maximum addition of 

 fields no longer occurs and the induct- 

 ance of the system is very much re- 

 duced. The special advantages of this 

 method of mounting arise from the fact 

 that parts having large differences of po- 

 tential are kept well separated. Al- 

 though the simple two-coil variometer 

 construction used in receiving coils will 

 give an inductance variation as large as 

 1 to 15 when insulation difficulties are 

 small, in the two coil form as applied to 

 high-powered transmitter the coils have 

 to be kept so far apart that the maxi- 

 mum inductance is only about twice the 

 minimum. With the sub-divided form 

 shown in this patent, however, heavy 

 currents can be carried and yet a con- 

 siderable inductance variation attained. 

 Fig. 6 shows an interference prevent- 

 er arrangement patented in 1915 by T. 

 B. Miller, specification No. 1127368. In 

 the ordinary interference preventer of 

 Fessenden two primary circuits con- 

 nected to the antenna act on two oppos- 

 ing secondaries ; one primary is adjusted 

 to receive the desired signals selectively 

 and to impress them upon the detector, 

 while undesired signals are caused to af- 

 fect both branches of the circuit equally 

 and oppositely and so produce no final 

 effect. The circuit of Fig. 6 differs 

 from this earlier arrangement in that a 

 single antenna primary circuit is used 

 with two secondaries and two detectors, 



Interference preventer 



and the neutralization of interfering sig- 

 nals is accomplished by opposing their 

 effects in the telephone circuits. 



